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Introduction

Respiratory failure is a condition in which the respiratory system fails in one or both of its gas-exchanging functions; that is, oxygenation of, and carbon dioxide elimination from, mixed venous (pulmonary arterial) blood. Hence, respiratory failure is a syndrome rather than a disease. Many diseases result in respiratory failure, as discussed elsewhere in this volume.

Respiratory failure may be acute or chronic. The clinical presentations of patients with acute and chronic respiratory failure usually are quite different. While acute respiratory failure is characterized by life-threatening derangements in arterial blood gases and acid–base status, the manifestations of chronic respiratory failure are more indolent and may be clinically inapparent.

Although the causes of respiratory failure are diverse, common underlying pathophysiological mechanisms and management strategies merit a general discussion. This chapter begins with a focus on the definition of respiratory failure and underscores distinctions between acute and chronic varieties. Hypoxemic and hypercapnic respiratory failure are described, and the pathophysiological underpinnings of each type are reviewed. The concepts of ventilatory supply and demand are considered before an overview of the many categories of disease that result in respiratory failure. Finally, an approach to clinical evaluation and management is outlined, followed by a summary of complications, comments on prognosis, and consideration of sites of care for survivors of respiratory failure who remain “chronically critically ill.”

Classification of Respiratory Failure

As noted previously, respiratory failure is characterized by inadequate blood oxygenation or carbon dioxide removal. “Adequacy” is defined by tissue requirements for oxygen uptake and carbon dioxide elimination. In the absence of bedside techniques for direct measurement of these metabolic parameters, clinicians must rely on arterial blood gas values.

Respiratory failure may be classified as hypercapnic or hypoxemic (Fig. 139-1). Hypercapnic respiratory failure is defined as an arterial PCO2 (PaCO2) greater than 45 mm Hg. Hypoxemic respiratory failure is defined as an arterial PO2 (PaO2) less than 55 mm Hg when the fraction of oxygen in inspired air (FiO2) is 0.60 or greater. In many cases, hypercapnic and hypoxemic respiratory failure coexist. Disorders that initially cause hypoxemia may be complicated by respiratory pump failure (see below) and hypercapnia. Conversely, diseases that produce respiratory pump failure are frequently complicated by hypoxemia due to secondary pulmonary parenchymal processes (e.g., pneumonia or atelectasis) or vascular disorders (e.g., pulmonary embolism).

Figure 139-1

Classification of respiratory failure. Although depicted as distinct entities, hypercapnic and hypoxemic respiratory failure frequently coexist. Either may be acute or chronic.

Distinctions between acute and chronic respiratory failure are summarized in Table 139-1. In general, acute hypercapnic respiratory failure is defined as a PaCO2 greater than 45 mm Hg with accompanying acidemia (pH less than 7.30)....

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